Sink basin



FIG. 1 is a top perspective view of a sink basin, showing my new design;

FIG. 2 is front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top plan view thereof;

FIG. 5 is a right side view thereof; and,

FIG. 6 is a left side view thereof.

The broken line showing is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a sink basin, as shown. 